The use of small, bidirectional DC inductive motors is relatively widespread, finding use in a variety of machines and equipment and particularly in various applications within automotive vehicles. One such common application is the motor which drives the wiper or wipers on the window or windshield of automotive vehicles. In the control of such motors, it is often an objective to provide accurate and reliable control of the motor utilizing drive control circuitry which is relatively durable and inexpensive. Although the invention will be described in the context of its application to a wiper motor, it will be understood that its applicability extends to those small bidirectional DC inductive motors which drive other loads as well.
A relatively traditional wiper system for the window of an automotive vehicle is depicted in FIG. 1 herein. That system includes a two-speed wiper motor 10, a cammed Run switch 112, a cammed Park switch 114, relatively conventional drive and control electronics 116 which employed bipolar transistors and electromechanical relays, several ganged high-current wiper mode switches, generally represented as 118 and a potentiometer 120 for adjusting the delay interval for intermittent operation. The cammed Run switch 112 carries the current of wiper motor 110 while the wiper, or wipers, represented by reference numeral 122 is sweeping across the windshield. The cammed Park switch 114 is used to reverse the direction of rotation of wiper motor 110 which in turn brings the wiper 122 to the "Depressed Park" position. The forward and return sweeping motion of the wiper 122 across a windshield is performed in the linkage between it and the wiper motor 110 such that the motor is normally only rotated in a single direction, but is reversed to move the wiper 122 into the "Depressed Park" position. Other wiper systems exist in which the linkage to the wipers and the control of the motor is such that the direction of rotation of the motor is reversed for the forward and return sweeping motion.
A recently-developed wiper control system is described in U.S. Pat. No. 4,578,591 for CONTROL CIRCUIT FOR AUTOMOTIVE ACCESSORY SYSTEM by Floyd et al, issued March 26, 1986, and having the same assignee as the present invention. That application describes in somewhat greater detail the form of motor drive circuitry that has existed in the prior art. Further, that application describes a system in which signals from input switches are used in conjunction with a microcomputer-based controller for directing the control of the windshield wipers. The control signals provided by the microcomputer were conducted via a serial multiplex link and respective remote multiplex (REMUX) controllers or slave units to the associated motor drive circuitry.
Still further, U.S. Pat. No. 4,454,454 to Valentine for MOSFET "H" SWITCH CIRCUIT FOR A DC MOTOR describes what is characterized as an "improved MOSFET H switch circuit for a DC motor". That circuit employs four power MOSFETS connected in an "H" switch, or bridge, configuration for providing bidirectional control of a fractional horsepower DC motor, as for instance used to drive windshield wipers. That motor drive circuit is characterized as providing various advantages over the use of bipolar transistors for reversing the motor supply voltage for bidirectional control and/or for speed control of the motor. Indeed, the utilization of power MOSFETS in an H-bridge configuration does appear to provide advantages over the utilization of other types of switching devices. While that patent does disclose the use of power MOSFETS in an H-bridge configuration for controlling the direction of rotation and further provides for the use of pulse-width modulated control signals for regulating speed, the manner in which the power MOSFETS are controlled appears to provide little or no opportunity for braking the motor, if desired. Moreover, it does not appear that full consideration has been given to the need to minimize the potential for cross-conduction between power MOSFETS connected to a common terminal of the motor nor is there a full accommodation for or recognition of the effects of the inductance in the circuit on the control dynamics of the motor and on the integrity of the power MOSFETS.
Accordingly, it is a principal object of the present invention to provide an improved motor drive circuit for a bidirectional inductive DC motor which employs power MOSFETS in an H-bridge configuration and which optimizes the utility of the control and the integrity of the drive circuitry.
It is a further object of the present invention to provide an improved motor drive circuit of the type mentioned and which readily accommodates control signals received from a microcomputer for bidirectional control and for speed regulation using pulse-width modulation. Included within this object is the provision of a drive control circuit which readily accommodates a braking capability. Also within this object is the provision of a motor drive circuit which minimizes potentially-destructive cross-conduction between power MOSFETS and which accommodates the inductive characteristics of the motor and leads in an optimized manner.